In the past five years, the use of fluorescent-activated flow cytometry with specific activating antibodies has allowed investigators to directly monitor platelet activation and subsequent microaggregate formation. While accurate and specific, flow cytometry is extremely expensive, requiring an expensive machine and maintenance, and a dedicated technician. Platelet assay methods using other well-known assay systems, such as ELISA, radioimmunoassay (RIA) and Western blot analysis have been described in Berman et al., Methods in Enzymology 169:314, 1989. Berman et al. discuss an ELISA assay which involves fixing platelets and placing the fixed platelets directly in a microtiter plate for the purpose of screening for activation specific antibodies.
Current assays for monitoring platelet activation in patients who may be at risk for thrombosis or thrombosis-related conditions involve indirect assessment of this activation by measuring systemic released platelet products (factors present in the circulation). Such products include platelet factor 4 and .beta.-thromboglobulin, or the thromboxane B.sub.2, a stable product of platelet activating factor thromboxane A.sub.2. Unfortunately, circulating levels of these factors can undergo metabolically-produced changes, and as such these factors are considered indirect measures of platelet activation. As such, a need remains for a direct platelet activation assay which can be carried out on patient samples without undue complexity or expense.